Microelectromechanical system (“MEMS”) resonators are small electromechanical structures that vibrate at high frequencies and are often used for timing references, signal filtering, mass sensing, biological sensing, motion sensing, and other applications. MEMS resonators are considered a common alternative to quartz timing devices. In general, quartz resonators have a good quality factor and piezoelectric coupling, but one limitation for quartz resonators is that they are difficult to design in smaller sizes.
Typically, MEMS resonators are made of silicon using lithography based manufacturing processes and wafer level processing techniques. Designers have found that pure silicon resonators often demonstrate very high quality factors comparable to quartz crystals, for example, as described in Non-patent document 1 (identified below). However, bare silicon is not piezoelectric and pure silicon resonators have high motional impedance making them unsuitable to replace quartz resonators in many applications.
In order to lower the motional impedance of MEMS resonator, some designs have added piezoelectric material, such as a layer of thin film of aluminum nitride (AlN), as described in Non-patent document 2 (identified below), for example. In a typical piezoelectric MEMS resonator, a thin film of molybdenum may be sputtered onto the silicon followed by a layer of AlN and an additional layer of molybdenum. After thin film deposition, the metal layers, the AlN layer and the silicon are etched to form the resonator shape. With the resulting design, the lower and upper layers of molybdenum serve as electrodes to excite and detect the mechanical vibrations of the resonator.
FIG. 1 illustrates a conventional micromechanical bulk acoustic resonator. As shown, the bulk acoustic resonator includes silicon layers 11 and 13 with an insulator 12 disposed therebetween. Moreover, two metal layers 14, 16 are disposed on top of silicon substrate 13 with a piezoelectric film 15 disposed therebetween. The metal layer 16 forms conductive electrodes 16 that are coupled by way of tethers 17 to conduct pads 16a and 16b. One limitation with this design is that the addition of the piezoelectric film 15 and the metal layers 14 and 16 on top of the silicon 13 breaks the symmetry of the resonator 10. In other words, the top of silicon is dissimilar to the bottom of silicon. The asymmetrical design causes vibrations in the thickness direction of the resonator that result in energy leakage out of the resonator.
FIGS. 2A and 2B illustrate a comparison of vibration between a pure silicon resonator and a silicon resonator with thin films deposited on a top surface of the silicon layer. In both FIGS. 2A and 2B, the dashed outline represents the device in its original position with no vibration. FIG. 2A illustrates a pure silicon resonator 110 having an anchor center point 120. In vibration mode, the device 110 expands and contracts as shown in the two images of FIG. 2A, but there is no movement in the z direction, i.e., the anchor center point 120 does not move up or down while vibrating. FIG. 2B illustrates a resonator design that includes thin films (e.g., the piezoelectric layer and metal layers) 112 disposed on top of the silicon substrate. The piezoelectric and metal films have different elastic modulus and density than silicon. Because the symmetry is broken, the resonator bends and there will be vibration movement in the z direction, i.e., the anchor center point 120 will move up or down while vibrating. As a result, piezoelectric MEMS resonator designs, such as those shown in FIGS. 1 and 2B, will typically have a quality factor that is about an order of magnitude lower than bare silicon resonators, such as the device shown in FIG. 2A, at the same frequency. The low quality factor of the piezoelectric MEMS resonator designs increases the noise in oscillator applications and increases the motional impedance.
One design that attempts to overcome the low quality factor of piezoelectric MEMS resonators is to increase the size of the resonator by using a higher order overtone design, for example, as described in Patent document 1 (identified below). While a higher order overtone design directly decreases the motional resistance, it also increases the size of the resonator. Moreover, since the manufacturing cost of the resonator is proportional to the size, the larger resonator size is not preferred. In addition, even for larger resonators, the low motional impedance is still not sufficient for low noise oscillator applications and a higher quality factor is required.                Non patent document 1: V. Kaajakari, T. Mattila, A. Oja, J. Kiihamaki, and H. Seppa, “Square-extensional mode single-crystal silicon micromechanical resonator for low phase noise oscillator applications”, IEEE Electron Device Letters, Vol. 25, No. 4, pp. 173-175, April 2004.        Non patent document 2: G. Piazza, P. J. Stephanou, A. P. Pisano, “Piezoelectric Aluminum Nitride Vibrating Contour-Mode MEMS Resonators”, Journal of MicroElectro Mechanical Systems, vol. 15, no. 6, pp. 1406-1418, December 2006.        Patent document 1: U.S. Pat. No. 7,924,119.        